The present invention relates to electrical timing devices for controlling the mode and duration of operation of a domestic appliance such as a clothes dryer or microwave cooking oven. Appliances of this type for household use commonly employ a plurality of electrical switches sequentially operated by a rotating cam drum or similar mechanism. Typically the cam drum is rotated in timed advance for operating the switches by a suitable speed reducer means commonly driven by a small sub-fractional horse power synchronous timing motor, and, cam followers actuate the various switches.
Such devices are known as electromechanical timers and are employed in household appliances produced commercially in high volume production as a cost effective and competitive alternative to a more sophisticated programmable all-electronic switching mechanism in view of the cost of the solid state switching devices and microprocessors required to provide the programming for the various selected modes of operation and time durations. All-electronic programmer timers require solid state switching devices capable of handling the current load required for the appliance load function as for example the magnetron in a microwave cooking oven. Solid state switching devices capable of handling the appliance load current are prohibitively expensive for commercial volume production of household appliances. Thus, although all-electronic programmer timers may provide sophisticated combinations of control functions by virtue of the program storage capability of the microprocessor memory, such all-electronic systems are expensive to produce. In addition, all-electronic programmer timers, although capable of providing the user with a wide variety of control functions, often present a confusing array of user inputs and are thus more difficult to operate by users unfamiliar with digitally encoded control inputs.
Thus it has been desired to improve the program control capability of electro-mechanical programmers. In designing electro-mechanical programmer timers for appliances such as microwave cooking ovens, it has been desired to provide the user with a selection of power levels of magnetron operation in addition to the selection of the duration of the cooking program interval. Heretofore, microwave cooking ovens have provided thereon separate selector switches for the user to select the power level of operation and for the selection of the duration of program interval. For example a rotary dial knob has been provided for setting the program time interval and a separate push button or slide type switch has been provided for the selection of the mode or power level of operation.
Known mechanisms for providing plural modes or power levels of magnetron operation in a microwave oven have employed separate cam drums and timing motors for advancing the cams at different rates for providing fractional duty cycle pulsing of the magnetron to thereby provide less than full "ON" operation of the magnetron during the desired program interval. These types of programmer timers have been found to be costly to manufacture and have often resulted in improper cooking where the user sets the program interval timer for the desired time duration and inadvertently fails to select the correct power level of operation. Therefore it has long been desired to find a way or means of providing a microwave cooking oven having an electromechanical programmer timer low in manufacturing cost which provides multiple power level operation and to provide such an oven with simplified controls for user operation.